The invention relates to a method refining molten glass, in which refining gas is generated by refining agents in the molten glass.
In the context of molten glass, the term refining is understood to mean the removal of gas bubbles from the molten material. To achieve the maximum possible freedom from foreign gases and bubbles, it is necessary for the molten mix to be thoroughly mixed and degassed.
The behaviour of gases or bubbles in molten glass and the way in which they are removed are described, for example, in xe2x80x9cGlautechnische Fabrikationsfeblerxe2x80x9d [Glass Engineering Manufacturing Errors], edited by B. Jebsen-Marwedel and R. Bruckner, 3rd edition, 1990, springer Vertag, on pages 195 ff.
In general terms, two refining principles which differ significantly through the way in which the refining gas is generated are known.
In the physical refining methods, by way of example the viscosity of the molten glass is reduced by increasing the temperature. Therefore, to reduce the viscosity higher temperatures of the molten glass are set during the refining than in the melting and cooling-down period. The higher the refining temperature selected can be, the more effective the removal of bubbles from the molten material. If possible, the viscosity of the molten material should be below 102 dPas. However, the maximum permissible refining temperature is limited by the ability of the wall material of the melting unit used to withstand such temperatures and, where Pt alloys are used, is at most 1600xc2x0 C. while where refractory bricks are used, it is at most 1650xc2x0 C. to 1700xc2x0 C. 
Further physical refining methods are distinguished by the fact that the flow inside the molten glass is influenced by causing mechanical movements of the molten material by poling or by blowing in gas, by the fact that the molten material is mechanically vibrated by the action of sound or ultrasound or bubbles are removed by means of centrifuging. Furthermore, vacuum (vacuum refining) or pressure (high-pressure refining) is employed, or intensified bubble nucleation is initiated by roughening of the surface.
Most commonly, chemical refining methods are employed. The principle of such methods consists in adding to the molten material compounds which decompose and split off gases, or compounds which are volatile at elevated temperatures, or compounds which release gases in an equilibrium reaction at elevated temperatures.
The first group of compounds includes, for example, sodium sulphate, which is used for refining soda-lime glass materials. In this case, SO2 and O2 are released in a temperature range of from 1300xc2x0 C. to 1450xc2x0 C., with a maximum at 1380xc2x0 C. This temperature range approximately corresponds to the refining range for glass materials of this type.
Compounds which are volatile at high refining temperatures owing to their vapour pressure and act in this way include halides. By way of example, a series of borosilicate glass materials are refined using NaCl.
Finally, the last group of substances comprises the so-called redox refining agents, such as for example arsenic oxide and antimony oxide. In this case, the redox refining agents used are polyvalent ions which can occur in at least two oxidation states which are in a temperature-dependent equilibrium with respect to one another, a gas, generally oxygen, being released at high temperatures.
The redox equilibrium of the substance dissolved in the molten material can be demonstrated with reference to the example of arsenic oxide using the equation (I)
AS2O5⇄As2O3+O2↑xe2x80x83xe2x80x83(I).
The equilibrium constant K for (I) can be formulated as shown in equation (II):                               K          ⁡                      (            T            )                          =                                            As                                                xe2x80x83                                2                                                                          a                        ⁢                                          O                3                            ·                              PO                                                      xe2x80x83                                    2                                                                                        As              2                                                            a                        ⁢                          xe2x80x83                        ⁢                          O              5                                                                    (          II          )                .            
In this equation, aAs2O3 and aAs2O5 denote the activities of arsenic trioxide and arsenic pentoxide, respectively, and pO2 denotes the fugacity of oxygen.
The equilibrium constant K is highly temperature-dependent, and a defined oxygen fugacity pO2 can be set using the temperature and the activity of the oxidic arsenic compounds.
For the chemical refining, it is possible to distinguish between substantially three refining effects:
1) a primary refining effect, under which the gases which are formed during decomposition of the refining agents added, for example oxygen from redox refining agents, diffuse into the bubbles which are formed during the decomposition of the mix, for example CO2, N2, H2O, NO, NO2 bubbles;
2) a secondary refining effect, under which gases are removed from the molten glass, involving the spontaneous formation of gas bubbles by the refining agents added, e.g. O2 bubbles from redox refining agents. Foreign gases, such as CO2, H2O, N2, NO, NO2, can diffuse into these refining bubbles even if their partial pressure is below 105 Pa, and
3) a so-called resorption effect, under which bubbles which have formed as described in 1) or 2), and in the event of a temperature reduction expanded bubbles of, for example, oxygen which are still in the molten material, are dissolved, for example in the case of the redox equilibrium (I) through a shift of the equilibrium towards the starting material.
For high-melting glass material which only have a viscosity of  less than 102 dPas above 1700xc2x0 C., the known refining agents, such as Na2SO4. NaCl, As2O5 or Sb2O5, are ineffective. The refining gases are released as early as during melting, and therefore the refining gases are no longer available for the secondary refining effect. Only the primary refining effect takes place. Standard redox refining agents, such as As2O5 or Sb2O5, are effective at releasing refining oxygen between 1150xc2x0 C. and 1500xc2x0 C., with a maximum at 1220xc2x0 C. to 1250xc2x0 C., the release of oxygen outside the refining temperature being substantially dependent on the glass composition and on the refining-agent composition (one or more refining agents). Particularly for high-melting glass materials, it is necessary to add larger amounts of refining agent than are actually required in order to achieve a refining effect at all. The large amounts of refining agent represent a particular drawback with arsenic oxide and antimony oxide, since these are highly toxic and expensive compounds. Moreover, the addition of refining agents may have an adverse effect on the properties of the glass material and may increase the production costs, since they are generally expensive compounds owing to the interaction with the tin float bath, arsenic oxide and antimony oxide cannot be used in the float glass process. The fact that high-melting glass materials only reach the viscosity of  less than 102 dPas, which is advantageous for the refining, at temperatures above those which are conventionally achievable means that such materials are difficult to refine or else effective refining is altogether impossible.
A series of patents have attempted to use SnO2, which releases its maximum level of refining gas at elevated temperatures as refining agent for high-melting glass materials.
By way of example, DE 196 03 698 C1 which corresponds to U.S. Pat. No. 5,770,535 issued on Jun. 23, 1998 to Brix, et al. and entitled xe2x80x9cAlkali-free aluminoborosilicate glass and its use,xe2x80x9d has disclosed the use of from 0.5 to 2.0% by weight SnO2 as refining agent for refining alkali-metal-free aluminoborosilicate glass, the refining of the molten glass being carried out at 1600xc2x0 C.
The use of from 0.02 to 1.0 mol % SnO2 and of from 0.02 to 0.5 mol % CeO2 as refining agents for refining aluminosilicate glass materials which can be chemically tempered is known from DE 196 16 633 C1, which corresponds to U.S. Pat. No. 5,895,768 issued on Apr. 20, 1999 to Speit and entitled xe2x80x9cChemically prestressable aluminosilicate glass and products made therefromxe2x80x9d. The refining is carried out at 1580xc2x0 C. in a Pt crucible.
The use of from 0.5 to 2.0% by weight SnO2, preferably together with nitrates, as refining agents for refining alkali-metal-free aluminoborosilicate glass is also known from DE 196 17 344 C1 with a refining temperature of 1620xc2x0 C. being set i a quartz crucible.
DE 197 39 912 C2 mentions the use of from 0.2 to 1% by weight SnO2 and 0.1 to 0.5% by weight CeO2, inter alia, as refining agents, and the extremely good refining action achieved with a combination of the two refining agents is pointed out. The aluminoborosilicate, glass claimed is refined at a temperature of 1620xc2x0 C. in a crucible made from SiO2 ceramic.
The maximum release of refining gas by SnO2 is in the upper range of the melting temperature which can conventionally be achieved. Therefore, in all these examples it is attempted to refine high-melting glass materials by metering in generally high levels of the refining agents, which often has an adverse effect on the product properties, such as in the case of SnO2 on the crystallization stability, or in the case of colouring oxides such as CeO2 on the colour locus of the product. In the case of high-melting glass materials, the fusion temperature lies in the maximum permissible temperature range for the tank furnaces made from ceramic refractory material or platinum.
For aluminosilicate glass and glass ceramic materials, the viscosity range of  less than 102 dPas which is advantageous for the refining is only reached at over 1700xc2x0 C., i.e. in a temperature range which cannot be reached using conventional melting technology and refining methods.
The invention is based on the object of finding a method for refining molten glass in which refining gas is generated by refining agents in the molten glass, which makes it possible to fully exploit the refining potential of known refining agents, which permits the use of novel refining agents, which improves or allows the refining of high-melting glass materials, in particular the refining of glass materials which only reach a viscosity of  less than 102 dPas at over 1700xc2x0 C., which reduces the tendency to reboiling, which eliminates or considerably reduces the use of toxic refining agents and which allows the refining agents to be metered in in smaller quantities (while maintaining a constant or even improved refining action). The release of refining gas by refining agents is to take place in a temperature range within which the viscosity of the molten glass is sufficiently low to allow the bubbles to rise quickly to the surface of the molten material.
Furthermore, the method is to allow a significant reduction in the refining time and/or a significantly smaller refining volume compared to the prior art.
This object is achieved by the fact that a method for refining molten glass materials in which refining gas is generated by refining agents in the molten glass is provided, in which method the molten glass is heated to a temperature of between 1650xc2x0 C. and 2800xc2x0 C. and the maximum release of refining gas by the refining agents takes place at over 1500xc2x0 C. and preferably over 1650xc2x0 C.
Advantageous modifications of the method form the subject matter of the dependent patent claims.
The advantages of the refining method according to the invention essentially consist in the fact that, unlike in the prior art, the refining potential of known refining agents is fully exploited. Improved and therefore more effective refining is achieved using standard amounts of known refining agents, or refining which is just as good as that achieved previously is obtained using smaller amounts of known refining agents than is customary, due to the higher refining temperature.
The method according to the invention also makes it possible to carry out a secondary refining for high-melting glass materials. The fact that the molten glass is heated to a temperature of between 1650xc2x0 C. and 2800xc2x0 C. and that the maximum release of refining gas by the refining agents takes place at over 1500xc2x0 C. means that spontaneous bubbles of refining gas are generated for the first time within this temperature range. The temperature range for the formation of bubbles of refining gas lies above the temperature range for the melting of the glass. The residual gases which remain in the molten glass after melting, such as for example CO2, NO2, SO2, H2O, can diffuse into the bubbles of refining gas, even if their partial pressure is already below 105 Pa. The reduction in the partial pressure of the residual gases leads to a significant reduction in the risk of the molten glass reboiling. Toxic refining agents which have hitherto been customary, such as As2O5 and Sb2O5, can be dispensed with altogether, or the amount of such agents can be reduced considerably. In general terms, significantly smaller amounts of the refining agents can be metered in, while maintaining a constant or even improved refining of the molten glass. Moreover, a considerable reduction in the refining time is achieved, and the refining volume can be reduced significantly.
In a preferred procedure, the molten glass for refining is heated to a temperature of between 1700xc2x0 C. and 2400xc2x0 C., the maximum generation of refining gas by the refining agents taking place at over 1600xc2x0 C., and preferably over 1700xc2x0 C.
In addition to the advantageous chemical refining, the physical refining which is improved as a result of the high temperatures also plays an important role. At the high temperatures involved, the viscosity of the molten material falls, so that the rate at which the bubbles rise is significantly increased. Thus, the rate at which the bubbles rise in molten glass which has been heated to 2400xc2x0 C. is approximately 100 times greater than in a corresponding molten material which has been heated to 1600xc2x0 C. This means that given a rate of rise which is 100 times higher, the residence time for the molten glass (refining time) can be reduced by a factor of 100.
Moreover, the diffusion of the foreign gases which are dissolved in the molten material (CO2, N2, . . . ) is higher at high temperatures than at conventional temperatures. Consequently, the degassing proceeds more quickly. The gases diffuse more quickly into refining bubbles. Furthermore, there is considerable convection at the high temperatures of the molten glass, ensuring that each volumetric element of the molten glass passes into that part of the molten glass which is close to the surface at regular intervals, and in that area the bubbles are expelled owing to the lifting forces acting on them. Moreover, the convection means that each volumetric element of the molten material is passed through the hottest regions of the refining unit, so that the refining agents can reveal their full potential.
The combination of all these effects, namely chemical refining, expansion of the bubbles owing to the high temperature, considerable convection and a high rising rate of the bubbles owing to the low viscosity, results in rapid and effective refining of the molten glass.
For example, the typical refining time for molten glass which is 50 cm deep and at a temperature of 1600xc2x0 C. in a melting unit with a capacity of 50 l is one day in order to remove all bubbles with a radius of less than 0.3 mm using their buoyancy. By contrast, a refining temperature of 2400xc2x0 C. results in a refining time of 5 minutes without taking the convection into account and of 2 minutes taking the convection into account.
Preferably, the viscosity of the molten glass is set at a level of less than 103 dPas, and particularly preferably at a level of less than 102 dPas. It has been found that although making it easier for the bubbles to rise up owing to the lower viscosity does, as expected, have a positive influence on the refining, this effect is not sufficient to dispense with the use of refining agents. For aluminosilicate glass and glass ceramic materials, a viscosity of less than 102 dPas is reached at over 1650xc2x0 C., often above 1700xc2x0 C., so that these glass materials can for the first time be subjected to effective, i.e. improved and time-saving, refining.
It has proven particularly advantageous if the method is carried out in such a way that the molten glass is set to the temperature at which the maximum release of refining gas by the refining agents takes place, i.e. a maximum of refining gas is generated and released using a minimum amount of refining agent, thus contributing to refining of the molten glass.
To carry out the refining method according to the invention as advantageously as possible, it is expedient if the refining agents added are redox compounds, in particular redox oxides, such as SnO2, CeO2, Fe2O3, ZnO, TiO2, V2O5, MoO3, WO3, Bi2O5, PrO2, Sm2O3, Nb2O5, Eu2O3, TbO2 and/or Yb2O3. In principle, all redox compounds which release the maximum amount of refining gas at over 1500xc2x0 C., in particular over 1600xc2x0 C., are suitable.
Some rare earth oxides which also release a maximum amount of refining gas at over 1600xc2x0 C. are also of interest for the redox refining.
It has been possible to demonstrate that with the known refining agents such as SnO2 and CeO2, the maximum release of refining gas takes place at temperatures over 1500xc2x0 C., and that surprisingly redox oxides, such as Fe2O3, ZnO, TiO2, V2O5, MoO3, WO3, Bi2O5, PrO2, Sm2O3, Nb2O5, Eu2O3, TbO2 and/or Yb2O3, can be used to equally good effect as refining agents, the maximum release of refining oxygen from these oxides likewise taking place at over 1500xc2x0 C.
Table 1 gives a number of examples of such redox compounds, as well as the temperature range within which the oxygen is released. The temperatures at which oxygen is released from the redox compounds also depend on the composition of the glass material.
The temperature ranges shown in Table 1 were determined using aluminosilicate glass materials.
The invention is not restricted to these redox compounds or to polyvalent redox compounds. Compounds which release oxygen at temperatures of between 1600xc2x0 C. and 2400xc2x0 C. and are converted into the metallic form, such as for example ZnO, SnO, Sb2O3, As2O3 and Bi2O3, are also suitable for redox refining.
Which redox compound is used as the refining agent depends on the other requirements imposed on the glass.
Many of the redox compounds colour the glass. The fact that, according to the claimed method, even small amounts of refining agents, in some cases  less than 0.2% by weight, considerably improve the refining has a positive effect in this connection.
In addition to the colouring effect, some of the rare earth oxides are very expensive and are only suitable under exceptional circumstances.
The wide range of redox compounds which can be used means that with the claimed method it is in many cases possible to dispense altogether with the use of the toxic antimony-containing and/or arsenic-containing refining agents.
In addition to the redox refining agents, it is also possible to use inorganic salts as refining agents for the high-temperature refining, which salts have a vapour pressure of greater than 105 Pa at over 1500xc2x0 C., in particular over 1600xc2x0 C. Preferably, the inorganic salts added to the mix as refining agents are halides.
As described above, the refining action of the halides resides in the fact that they pass into the gaseous state. Examples of chlorides which release maximum amounts of refining gas at over 1500xc2x0 C., in particular over 1600xc2x0 C., are KCl, CaCl2, BaCl2, LaCl3, CeCl3, YbCl2, ErCl3 and PrCl3. In addition to the chlorides, in particular a large number of fluorides have a vapour pressure of  greater than 105 Pa at over 1500xc2x0 C., such as for example LiF, NaF, KF, ZnF2, MgF2, BaF2, CeF2 or a series of rare earth fluorides.
A number of bromides also have a vapour pressure of  greater than 105 Pa at over 1500xc2x0 C. and can in principle be used according to the invention as refining agents. However, where possible the use of such bromides should be avoided for health and safety reasons.
During halide refining, it should be ensured that in each case the halide with the lowest vapour pressure forms and evaporates first, irrespective of the compound in which the halide was added to the mix. For example, if the glass contains lithium oxide and if KCl is added as the refining agent, LiCl evaporates at approximately 1350xc2x0 C., while the KCl does not evaporate at over 1500xc2x0 C. The chlorine can also escape from the molten glass as HCl.
The abovementioned halide refining agents can only exhibit their refining action at over 1500xc2x0 C. if the glass does not contain any components which are able to form halides with a lower evaporation temperature.
The refining action of the halides also depends on how highly soluble the halides are in the glass material.
It is also advantageous if the refining agents added are inorganic salts which decompose, releasing refining gas, at over 1500xc2x0 C., in particular over 1600xc2x0 C., and the decomposition products have a gas pressure of greater than 105 Pa. The inorganic salts used preferably contain oxo anions, in particular sulphates. For example, pure Na2SO4 reaches a gas pressure of greater than 105 Pa at approximately 1850xc2x0 C. With sulphates, decomposition takes place so as to form SO2 and O2.
Examples of preferred sulphates with decomposition temperatures of over 1500xc2x0 C. are K2SO4, MgSO4, CaSO4, SrSO4, BaSO4 and La2 (SO4)3. The method is not restricted to the sulphates listed above. In the case of sulphate refining too, effective refining can only be achieved if the solubility of the sulphates in the glass material which is to be refined is sufficiently high.
Particularly in the case of aluminosilicate glass materials, the solubility of the sulphates is relatively low, and therefore the sulphates can frequently only be used in combination with other refining agents.
The release of refining gas is preferably established by one refining agent or by a combination of a plurality of refining agents.
It is preferable to add non-toxic refining agents.
In a further, preferred procedure, the refining is assisted by physical refining methods as mentioned in the introduction.
The molten glass is preferably heated in a forcibly cooled crucible or a forcibly cooled tank furnace. It is particularly preferable for the molten glass to be heated in a forcibly cooled skull crucible by high-frequency means.
Glass materials which have a particularly high melting temperature and only reach a viscosity of  less than 102 dPas at over 1650xc2x0 C. are refined effectively by means of the method according to the invention.
The amount of refining agent required lies in the range from 0.01 to 3% by weight and is therefore dependent on the refining temperature and the refining time. The optimum redox refining agent or a combination of optimum redox refining agents can be selected from Table 1 according to the required refining temperature. Conversely, Table 1 offers a point of reference for determining the refining temperature at which the maximum release of refining gas by the refining agents takes place. It is then possible to use simple experiments to determine the optimum refining temperature for each molten glass composition and for a specific refining agent or for a plurality of specific refining agents. The viscosity of the molten material plays an important role in selecting the refining agents. For effective refining, the viscosity should be less than 103 dPas, and preferably less than 102 dPas. The lower the viscosity, the more positive this is for the refining. The energy costs required to heat the high-temperature refining part are to be regarded as a factor limiting the extent to which this fact can be optimized, since these costs rise as the refining temperature increases, owing to the rising energy losses through the wall of the melting unit.
The energy loss at the high melting temperatures can be kept at a low level because the dimensions of the refining crucible required can be small, owing to the high refining rate.
In the case of glass materials with highly volatile constituents, it should be noted that the evaporation of these highly volatile components increases as the refining temperature rises.